1. Field of the Invention
The present invention relates to a memory apparatus using tunnel current techniques, which may be termed a scanning tunneling microscope (STM) memory apparatus, comprising a memory device, which includes tunnel current probes and a recording element, and a signal processing circuit for writing/reading data in/from the recording element.
2. Description of the Related Art
A probe with a pointed tip portion is brought toward an electrically conductive substrate at a distance of several nm, and a voltage is applied across the probe and the substrate, so that a tunnel current flows therebetween by virtue of tunnel effect. The value of the tunnel current varies dramatically in accordance with the change in distance between the probe and the substrate.
An STM utilizes the characteristic of the tunnel current, and allows atomic-level observation of the surface configuration of a substrate. In the STM, a probe is caused to scan over the surface of the substrate, and the distance between the probe and the substrate is controlled. Thus, a three-dimensional image, representative of the surface configuration of the substrate is obtained. For example, while the distance between the probe and the substrate is controlled so as to keep the tunnel current constant, the probe is caused to scan over the surface of the substrate. Based on the positional data of the probe obtained during the scan, an STM image is obtained. The tunnel current flowing between the probe and the substrate mainly depends on the work functions of materials of the probe and the substrate. Thus, the tunnel current produced while the probe is caused to scan is changed by the data recorded on the substrate in the form of projections and recesses, the kind of atoms of the substrate, or the charge trapped on the surface of the substrate.
It has been proposed to describe a pattern directly on a silicon substrate with use of the STM. For example, Journal of Vacuum Science & Technology, McCord, "Lithography with the Scanning Tunneling Microscope", B Vol. 4, No. 1, pp. 86-88, Jan./Feb. 1986 (M. A. McCord, et al.) reports that tracks were formed on a metal deposition film on a silicon substrate after the film was STM-scanned with energy of 10 eV. Also this document reports that lines were written on a 10 nm docoscenoic aicd Langmuir-Blodgett (LB) film by means of a beam of 25 V and 12 nA.
Furthermore, an article by Akamine, Albrecht, Zdeblick and Quate, "MICROFABRICATED SCANNING TUNNELING MICROSCOPE" (IEEE Electron Device Letters, Vol. 10, No. 11, pages 490-492, November 1989) reported on a micro STM wherein an end portion of a cantilever is provided with a probe having a pointed tip portion. This cantilever is driven by two piezoelectric elements which are formed of zinc oxide (ZnO). The cantilever is formed by an integrated circuit (IC) process so as to have the size of 20 .mu.m.times.200 .mu.m.times.5 .mu.m. Also, the probe is formed by an IC process in which deposition is performed by using a mask with a hole of about 3 .mu.m diameter.